Why do so copious medication result in drowsiness?

I'm just wondering why this is such a adjectives side effect.Why do so copious medication result in drowsiness?
I guess sometimes, it's because those drugs cross the blood brain barrier?
Because from what i've read contained by some articles, certain drugs that don't exact drowsiness are those that do not cross the blood brain barrier.
I might be wrong though.
Somnolence (or "drowsiness", or "hypersomnia") is a state of near-sleep, a strong desire for sleep, or sleeping unusually long period. It has two distinct meaning, referring both to the usual state preceding falling asleep, and the chronic condition referring to being contained by that state independent of a circadian rhythm. The disorder characterised by the latter condition is most commonly associated with users of prescription hypnotics, such as Mirtazapine or Zolpidem.
It is considered a poorer impairment of consciousness than stupor or coma.